Structure of Bacterial 3β/17β-Hydroxysteroid Dehydrogenase at 1.2 Å Resolution:  A Model for Multiple Steroid Recognition^†,‡

The enzyme 3β/17β-hydroxysteroid dehydrogenase (3β/17β-HSD) is a steroid-inducible component of the Gram-negative bacterium Comamonas testosteroni. It catalyzes the reversible reduction/dehydrogenation of the oxo/β-hydroxy groups at positions 3 and 17 of steroid compounds, including hormones and isobile acids. Crystallographic analysis at 1.2 Å resolution reveals the enzyme to have nearly identical subunits that form a tetramer with 222 symmetry. This is one of the largest oligomeric structures refined at this resolution. The subunit consists of a monomer with a single-domain structure built around a seven-stranded β-sheet flanked by six α-helices. The active site contains a Ser-Tyr-Lys triad, typical for short-chain dehydrogenases/reductases (SDR). Despite their highly diverse substrate specificities, SDR members show a close to identical folding pattern architectures and a common catalytic mechanism. In contrast to other SDR apostructures determined, the substrate binding loop is well-defined. Analysis of structure−activity relationships of catalytic cleft residues, docking analysis of substrates and inhibitors, and accessible surface analysis explains how 3β/17β-HSD accommodates steroid substrates of different conformations.